Well perforating and logging methods and apparatus



Oct. 30, 1956 J. H. CASTEL ET AL 2,768,684

WELL PERFORATING AND LOGGING METHODS AND APPARATUS Filed Feb. 20, 1952E9 99? will/Q7114 5 M .w Z m W 14 5 a W F.

W/ m M4 M 2 6U v w 6 L M 4 v United States Patent WELL PERFORATING ANDLOGGING METHODS AND APPARATUS Jacques H. Caste] and Henry C. Waters,Houston, Tex.,

asslgnors to Perforating Guns Atlas Corporation, Houston, Tex., acorporation of Delaware Application February 20, 1952, Serial No.272,5l76

14 Claims. (Cl. 164.5lj

The present invention relates to borehole logging methods and apparatus,and, more particularly, to improved methods and apparatus for thelocation of easing collars in cased boreholes in conjunction with otherlogging or drilling operations such as casing perforating and the like.

In completing a borehole in accordance with estab lished commercialpractice it is customary to take an electrical log of the boreholebefore the casing is set in order to determine the productive horizonsin the structure through which the borehole extends. After theelectrical log is taken the casing is set by connecting casing sectionstogether by means of screw threaded collars and, in most instances, acombined gamma ray and casing collar log is taken after the casing isset. This gamma ray log is similar in many respects to the electricallog taken before the casing is set and may be correlated with theelectrical log so that the position of the casing collars relative tothe desirable horizons shown in the electrical log is accuratelyestablished. Accordingly, in nearly all wells, complete information isavailable as to each productive horizon and the location of this horizonis determined by the adjacent casing collars which are used as referenceor bench marks.

As existing oil and gas formations become depleted and relatively fewerproductive horizons are being discovered, it becomes increasinglyimportant to exploit formations which are deeper in the earth, which arethinner and are of relatively less value. This is customarily done inexisting wells by perforating the well casing by means of various typesof perforating guns which fire bullets or jets into and through the Wellcasing at points opposite the desired horizons to open up the casing atthese points and permit the oil to flow into the well. In order to dothis some means must be provided to determine accurately the depth ofthe perforating gun before it is fired. While depth measuring sheavesare customarily used for this purpose, they are not sufiicientlyaccurate, due to stretching of the cable, variations due to temperature,density of the mud, and the like, to locate relatively thin formationswhich are only a few feet thick. However, the depth measuring sheave iscapable of determining the proximate depth so that it is only necessaryto tie in the position of the perforating gun with one of the adjacentcasing collars, after which the depth measuring sheave or odometer canbe readjusted for the correct depth of the adjacent casing collar andreference is then made to the original electrical log as correlated withthe gamma ray and easing collar log for the accurate location of theperforating gun relative to the desired horizon.

While it is desirable to provide a simple collar logging system whichmay be carried with the perforating gun as a unit and which may becontrolled over the conventional single conductor cable which supportsthe gun, it is absolutely essential that the collar locating apparatusbe incapable of firing the perforating gun during the initial collarlogging operation. Otherwise the casing will be perforated at the wrongplace and will require either 2,768,634 Patented Get. 30, 1956 anexpensive squeeze job of cementing, or, in some instances, the well maybe lost entirely. It is also essential to provide a collar locatingsystem and apparatus which is extremely rugged to withstand the terrificmechanical shocks to which it is subjected during the perforatingoperation, and also to withstand the tremendous hydrostatic pressureswhich are encountered in deep boreholes.

Accordingly, it is a primary object of the present invention to providea new and improved casing collar locating system which is of simple,economical, and rugged construction, is designed for operation on asingle conductor cable, and is adapted to operate in conjunction withother borehole instruments, particularly instruments of the perforatinggun type.

It is another object of the present invention to provide a combinedcasing collar locating and perforating gun.

provisionv of a new and improved casing collar'logging system which isparticularly adapted to prevent false casing collar indications whichmay be due to changes in the impedance of the earth return path andwhich may result from changes in the galvanic effect thereof.

Still another object of the present invention resides in the provisionof a combined casing collar logging and perforating gun firing systemwherein the casing collar logging apparatus may be controlled over thesame single conductor cable as the perforating gun but is effectivelyisolated from the gun so as positively to prevent false or prematurefiring of the gun during the casing collar logging operation.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the accompanying drawing, in which:

The Fig. l is a somewhat diagrammatical illustration of the improvedcasing collar logging system of the present invention shown inconjunction with a perforating gun apparatus and Fig. 2 is adiagrammatic illustration of an alternative embodiment of the inventionwherein a permanent record of casing collars is provided.

Referring now to the drawing, the equipment shown above the broken line10 in the drawing comprises the surface equipment located at the surfaceof the earth, and the equipment shown below the line 10 constitutes theborehole apparatus which is supported on a suitable single conductorcable 11. bodimentthe subsurface equipment comprises the collar loggingapparatus indicated generally at 12 and a perforatin-g gun andassociated equipment indicated generally at 13, which are securedtogether and supported by the cable 11. In this connection it will beunderstood that the cable 11 comprises a single central conductor 14 andthe usual armored sheath 15 and is carried on a suitable power-operateddrum or reel 16 10- cated at the surface of the earth whereby the cableis adapted to be fed out or reeled in so as to move the subsurfaceequipment longitudinally in the borehole. The cable 11 serveselectrically to connect the surface equipment and the doWn-the-holeequipment comprising the collar logging apparatus 12 and the perforatinggun appa- In the illustrated em-' ratus 13. While the armored sheath ofthe cable H is grounded at the earths surface through the supportingreel is, it will be understood that the subsurface equipment is moved upand down within the casing sections 21 and 22 in the borehole 2t? andthe sections 21 and 22 are filled with a drilling mud or fluid so thatthe return current path is primarily through the casing of thesubsurf-ace equipment, the drilling mud, the casing itself and throughthe earth to the ground connections made at the surface equipment. Aconventional odometer or other depth measuring device may be connectedto the reel 16 for automatically indicating or recording the length ofcable within the borehole at any given instant. As described in detailhereinafter, this odometer may be employed in conjunction with theapparatus of the present invention to provide an eflicient and accuratelocation of a particular casing collar adjacent the desired horizon.

The surface equipment comprises a suitable alternating current generator25 which is adapted to produce a relatively high-frequency alternatingcurrent, preferably in the order of 400 C. P. S., which is impressedupon the primary of a power transformer 26. The generator 25 maycomprise a vacuum tube oscillator of any conventional circuit design,such as plate to grid feedback, which is adjusted for the properoperating frequency and, in accordance with one feature of the presentinvention, is provided with an oscillator tube having a maximum poweroutput of a relatively small value, preferably in the order of one watt.As will be described in more detail hereinafter, it is particularlyimportant in accordance with the present invention to provide agenerator 25, the power output of which is limited to a relatively smallvalue and a vacuum tube oscillator is a convenient means of obtainingsuch power limitations.

The secondary winding of the transformer 26 is utilized to supply powerto a Wheatstone bridge circuit indicated generally at 27. Morespecifically, the bridge circuit 27 comprises the resistance arms 28,29, and 3t) and a coupling transformer 31, the primary of which isconnected as the fourth leg of the bridge 27. The secondary 32 of thecoupling transformer 31 is connected to ground as indicated by theconventional ground symbol 33, and the other end of the winding 32 isconnected through the selector switch 34 to the center conductor 14- ofthe cable 11. The voltage produced across the secondary of the powertransformer 26 produces current flow through the parallel connectedsides of the bridge, i. e., the resistors 28 and 29, which are inparallel with the primary winding of the transformer 31 and the resistor3%. As a result, an alternating current voltage is produced across thesecondary of the transformer 32 even though the bridge 27 is balanced.

In the subsurface equipment the center conductor 14 of the cable 11 isconnected through a high pressure insulating pin 35 to the primary of anisolating transformer 36, the other side of the primary winding of thetransformer 36 being grounded through the low resistance fuse 37 to thecasing of the collar logging unit 12. In this connection it will beunderstood that the armored sheath of the cable 11 is connected to thecasing of the collar logging unit 12 and the perforating gun unit 13 andboth of these casings are grounded both at the earths surface throughthe armored shield 15 of the cable 11 and through the drilling mudwithin the casing itself.

The collar locating unit 12 includes the single collar locator coil 40which comprises a single-layer solenoidwound coil, one end of which isconnected to the casing of the unit 12, as indicated at 38, so that oneend of the coil 40 is grounded, and the other end of the coil 40 isconnected to the secondary winding 39 of the transformer 36, the otherend of the winding 39 also being connected to ground through the casingof the unit 12. In this connection it will be understood that the collarlogging unit 12, which is shown somewhat diagrammatically in thedrawing, actually comprises a mandrel of magnetic material including thepole pieces 41 and 42 which are connected on either end of the yoke 43,the coil 40 being wound around the yoke 43 and being covered withsuitable insulating material to prevent damage thereto during use. Alsoit will be understood that the transformer 39 and the fuse 37 areconnected in the manner shown in the drawing within the hollow yoke 43by any suitable mechanical means. Preferably the fuse 37 is connected inthe circuit in the manner shown by means of suitable clip-typereceptacles so that the fuse 37 may be replaced when it is burned outduring the perforating gun firing operation as will be described in moredetail hereinafter.

Considering now the operation of the collar logging unit 12, it will beunderstood that the voltage produced across the secondary winding 32 ofthe coupling transformer 31 at the earths surface is transmitted overthe cable 11 and coupled through the transformer 36 to the secondary 39thereof so as to transmit a relatively high current through the collarlocating coil 40. This large 400 cycle current flowing in the coil 45sets up an alternating current magnetic field about this coil whichlinks the casing sections 21 and 22 and induces eddy currents therein.In this connection it will be understood that a single layer collarlocating coil 40 is desirable since the hydrostatic pressure encounteredin deep wells is very great and if a multi-l-ayer coil is used thispressure may be sufficient to crush the innermost layers of the coil,thus part-ing the insulation thereof and causing a short circuit of aportion or all of the coil.

Since the coil 40 is a single layer coil, for the reasons describedimmediately above and the length thereof is limited for practicalpurposes, it is obvious that the impedance of the coil 46 is relativelysmall. In order to produce a detectable change in the impedance of thecoil 40, it is necessary to provide a relatively large current flowthrough this coil and in accordance with the present invention theisolating transformer 35 is provided to produce a relatively largecurrent flow through the coil 40 without requiring a large current fiowthrough the single conductor cable 11. In this connection it will beunderstood that if a large current is transmitted over the cable 11greater interference due to noise voltages and the like is experienced.Also, if a low impedance collar locating coil is connected directly inseries with the cable 11 the change in inductance and resistance of thecable 11 as it is reeled in and out has more effect and tends to produceerroneous collar indications.

The impedance of the collar locating coil 40 remains substantially thesame so long as the collar locating unit 12 moves vertically in ahomogenous section of the casing. However, any metallic anomaly, such asthe casing collar 23, will cause a change in the impedance of the collarlocator coil 40 in part due to a change in the inductance of the coil 40and in part due to a change in the losses caused by the eddy currents inthe adjacent metallic structure. The impedance change of the coil 40 isreflected back through the isolating transformer 36 to the primarywinding thereof, which winding is connected in series with the secondarywinding 32 of the coupling transformer 31. As a result there is produceda corresponding change in current through the primary winding of thetransformer 31 which produces an unbalance of the bridge circuit 27.

The output of the bridge circuit 27 is connected to the full Waverectifier circuit including the rectifiers 45 and 46 which arerespectively connected to the R. C. load networks 47 and 48 connected inseries between the cathodes of the rectifiers 45 and 46 and the commonjunction of the networks 47 and 48 is connected to the junction of theresistor 28 and the primary of the transf mfir 31. Thus, when the bridge27 is balanced the rectifier 45 operates to produce a'voltage across theload network 47 which is of the same amplitude and is of oppositepolarity to the voltage produced across the network 48'by the rectifier46. The sum of the voltages produced across the networks 47 and 48is'impressed upon an ammeter 50 and coupling condenser 51, connected inseries relation. Thus the condenser 51 'is charged to the net D. C.voltage appearing across the series connected networks 47 and 48 and themeter 50 indicates the charging current for the condenser 51.

When the collar locating coil 40 traverses one of the casing collars 23,the impedance of this coil changes due both to the self-inductance ofthe coil and to the change in the losses caused by the eddy currentswithin the casing and the casing collar. This abrupt change in impedanceis reflected back through the series-connected transformers 36 and 31 tothe Wheatstone bridge 27 so as to produce an unbalance therein. As aresult, the voltage produced across the load network 48 changes so thatthe condenser is charged to a different value and the needle 52 on themeter 50 is moved so as to close the contacts 53 and 54, at the sametime indicating the presence of a casing collar. In this connection itwill be understood that the contact 53 is carried by the movable arm ofthe meter 50 and the contact 54 is fixed, the current range of the meterbeing so chosen as to close the contacts 53 and 54 when the bridge 27 isabruptly unbalanced in response to casing collar traversal.

When the contacts 53 and 54 are closed in the manner described above, acircuit is established from the battery 55 through the contacts 53 and54 to a buzzer circuit indicated generally at 56 so that an audiblesignal is produced which the operator can hear and which informs himthat acasing collar is being traversed. The depth reading on theodometer connected to the reel 16 may then be read and recorded at themoment the buzzer 56 is energized so that the exact location of thesubsurface equipment is obtained.

In order to provide a sufficiently large signal at the e'arths surfaceto operate the Wheatstone bridge 27 satisfactori ly while employingcollarloca'tor coil 40 of low impedance, it is necessary that theprimary winding of the transformer 36 be connected directly to groundfrom the center conductor of the cable 11 so that substantially theentire voltage transmitted down the cable will be impressed across theprimary of the transformer 36. On the other hand, it is necessarytoprovide a control'circuit for firing the perforating gun 13 over thesingle conductor cable 11 after the equipment has been accuratelypositioned at the desired horizon. In accordance with the presentinvention there is provided the low resistance high current fuse 37which is connected from the bottom end of the primary winding of thetransformer 36 to the casing of the collar logging unit 12 and hence toground. During the collar logging operation the fuse 37, whichpreferably has a resistance of a fraction of anohrn, is connected to thecasing of the unit 12 so that substantially no voltage is transmitted tothe perforating gun unit 13.

After the desired casing collars have been located by means of thecollar logging unit 12 and in accordance with the above-describedprocedure, it is then necessary to clear the .cable 11 so that theperforating gun 13 may befired in accordance with any conventionaltechnique. In this connection it will be understood that the collarlogging system of the present invention may be employed in conjunctionwith any conventional depth measuring equipment to locate the casingcollar or collars adjacent theldesired horizon after the subsurfaceequipment has been approximately positioned adjacent the horizon asindicated on the conventional depth measuring equipment. Accordingly,the collar logging unit is in some instances employed for a relativelyshort length of casing so that the, impedance changes which result fromreeling in and reeling out the cable 11 are of substantially no effect.

However, the collar logging unit may be employed to log a substantialportion of the casing in which case the in-' ductance of the cable willchange due to a reduced mumber of turns, and the resistance thereof alsochanges due to the change in length and temperature. However, thesechanges are relatively gradual and, while they will pro-.

duce an unbalance of the bridge 27, the unbalance is so gradual that thecharging current for the meter 50 is very small and the meter is notdeflected sufficiently to close the contacts 53., 54. On the other'hand,when a casing collar is traversed, the abrupt unbalance of the bridge 27causes a relatively large charging current to flow through the meter 50so that the contacts 53 and 54 are closed and the indicator circuit isenergized.

In order to disconnect the casing collar logging unit 12 so that theperforating gun equipment 13 may be fired, the selector switch 34 isthrown to the position in which the battery 60 at the earths surface isconnected to the upper end of the supporting cable 11. The battery 60 isperferably of suflicient power to produce a relatively high current flowover the cable 11, through the primary of the transformer 36, andthrough the fuse 37 so that this fuse is blown and the center conductorof the cable 11 is no longer connected to ground through this fuse.After the fuse 37 is burned out any suitable means may be employed tofire the perforating gun 13. For example, the potentiometer 61 may beadjusted so that the current transmitted over the cable 11 is raised toa sufliciently high value to detonate one of the firing caps 62 intheperforating gun 13, this cap being connected from the centerconductor to the casing of the perforating gun unit 13 and hence toground potential over the above described path. In this connection itwill be understood that any other suitable system may be employed forfiring the perforating gun unit 13 after the cable 11 is cleared. Forexample, a sequential system of firing the gun such as is shown anddescribed in detail in a copending application SerialNo. 219,480, filedApril 5, 1951, which is assigned to the same assignee as the presentinvention may be employed when the selector switch 34 is thrown to theperforating gun firing position.

In accordance with an alternative embodiment ,of the present invention,the location of casing collars may be performed automatically bysubstituting a relay 65 for the buzzer circuit 56 as shown in Fig. 2. Arecording depth counter 66 is employed, which is driven from theconventional odometer 67 connected to the reel equipment 16 by asuitable selsyn system so that it is in synchronism with the odometer ofthe winch truck. A paper tape 68 is run into the depth counter 66 andthe relay 65 is employed to actuate a lever which in turn prints on thepaper tape 68 the depth of the casing collar each time the relay isenergized in response to an unbalance of the Wheatstone bridge 27.Obviously, other systems may be employed to record the casing collardepth, as a function of the unbalance of the bridge circuit by means ofa suitable recorder, volt meter or ammeter.

From the foregoing it will be evident that the present inventionprovides a simple, economical and reliable collar logging unit forlocating casing collars which is exceptionally rugged and may beemployed in conjunction with perforating gun apparatus or other welllogging tools to position the tool accurately Within the casing. Asingle layer collar locating coil is provided to reduce the effects ofhydrostatic pressure and the relatively low output of the coil isstepped up by means of an isolating transformer and a relatively lowpower generator at'the surface is employed to detect changes in theimpedance of the coil. A vacuum tube oscillator circuit is employed as agenerator which is incapable of providing sufficient power to fire theperforating gun equipment so that false or premature firing of theperforating gun is positively prevented in the system of the presentinvention. Furthermore, the collar locating equipment is isolated bymeans 7 of an extremely low resistance fuse during the collar loggingoperation so that a perforating gun which employs low resistance firingcaps is not affected during the recording of casing collar locations andthe accurate positioning of the subsurface equipment.

While particular embodiments of the invention have been shown, it willbe understood, of course, that the invention is not limited thereto,since many modifications may be made and it is therefore contemplated bythe appended claims to cover any such modifications as fall within thetrue spirit and scope of the invention.

What is claimed and desired to be secured by Letters Patent is:

1. A casing collar locating system, comprising a collar locator coil, asingle conduct-or cable supporting said coil within a casing forlongitudinal movement therethrough, a bridge circuit at the earthssurface and including one winding of a coupling transformer in one armthereof, an isolating transformer, means connecting the other winding ofsaid coupling transformer, the single conductor of said cable and onewinding of said isolating transformer in series, means connecting saidcollar locating coil across the other winding of said isolatingtransformer, a source of high frequency alternating current forenergizing said bridge circuit, and means for indicating unbalance ofsaid bridge circuit resulting from a change in the impedance of saidcoil as said coil traverses a casing collar.

2. A casing collar locating system, comprising a singlelayer collarlocator coil, a single conductor cable supporting said coil Within acasing for longitudinal movement therethrough, a bridge circuit at theearths surface and including one winding of a coupling transformer inone arm thereof, an isolating transformer, means connecting the otherwinding of said coupling transformer, the single conductor of said cableand one winding of said isolating transformer in series, meansconnecting said collar locating coil across the other winding of saidisolating transformer, a source of high frequency alternating currentfor energizing said bidge circuit, means for rectifying the output ofsaid bridge circuit to provide a unidirectional control voltage inresponse to each unbalance thereof, and means controlled by said controlvoltage for producing an audible signal each time said bridge circuit isunbalanced due to a change in the impedance of said collar locating coilas said coil traverses a casing collar.

3. A casing collar locating system, comprising a singlelayer collarlocator coil, a single conductor cable supporting said coil within acasing for longitudinal movement therethrough, a bridge circuit at theearths surface and including one winding of a coupling transformer inone arm thereof, an isolating transformer, means connecting the otherwinding of said coupling transformer, the single conductor of said cableand one winding of said isolating transformer in series, meansconnecting said collar locating coil across the other winding of saidisolating transformer, a source of high frequency alternating currentfor energizing said bridge circuit, means for rectifying the output ofsaid bridge circuit to provide a unidirectional control voltage inresponse to each unbalance thereof, a meter having a movable contact onthe indicating arm thereof and an associated fixed contact, a condenserconnected in series with said meter, and means for impressing saidcontrol voltage on said series connected condenser and meter, wherebysaid indicating arm is deflected and said contacts closed each time saidbridge circuit is unbalanced due to a change in the impedance of saidcollar locating coil as said coil traverses a casing collar.

4. A casing collar locating system, comprising a singlelayer collarlocator coil, a single conductor cable supporting said coil within acasing for longitudinal movement therethrough, a bridge circuit at theearths surface and including one winding of a coupling transformer inone arm thereof, an isolating transformer, means connecting the otherwinding of said coupling transformer, the single conductor of said cableand one winding of said isolating transformer in series, meansconnecting said collar locating coil across the other winding of saidisolating transformer, asource of high frequency alternating current forenergizing said bridge circuit, means for rectifying the output of saidbridge circuit to provide a unidirectional control voltage in responseto each unbalance thereof, a meter having a movable contact on theindicating arm thereof and an associated fixed contact, a condenserconnected in series with said meter, means for impressing said controlvoltage on said series connected condenser and meter, whereby saidindicating arm is deflected and said contacts closed each time saidbridge circuit is unbalanced due to a change in the impedance of saidcollar locating coil as said coil traverses a casing collar, and meansoperative when said contacts are closed for producing an audible signal.

5. A casing collar locating system, comprising a singlelayer collarlocator coil, a single conductor cable supporting said coil within acasing for longitudinal movement therethrough, a bridge circuit at theearths surface and including one winding of a coupling transformer inone arm thereof, an isolating transformer, means connecting the otherwinding of said coupling transformer, the single conductor of said cableand one winding of said isolating transformer in series, meansconnecting said collar locating coil across the other winding of saidisolating transformer, a source of high frequency alternating currentfor energizing said bridge circuit, means for rectifying the output ofsaid bridge circuit to provide a unidirectional control voltage inresponse to each unbalance thereof, a recording depth meter controlledin synchronism with the position of said collar locating coil, and meanscontrolled by said control voltage for actuating said meter each timesaid bridge circuit is unbalanced due to a change in the impedance ofsaid coil as said coil traverses a casing collar to record the collardepth.

6. A combined casing collar locating and perforating gun firing system,comprising a perforating gun having firing means responsive to apredetermined minimum current, an inductive element, a single conductorcable supporting said inductive element and said gun within a casing forlongitudinal movement therethrough, a bridge circuit at the earthssurface, means including said cable for connecting said inductiveelement in one arm of said bridge circuit, a source of high frequencycurrent for energizing said bridge circuit, the maximum power output ofsaid source being many times less than the power required to produce acurrent flow through said inductive element of sufiicient magnitude toexcite said firing means, means for indicating unbalance of said bridgecircuit resulting from a change in the impedance of said inductiveelement as said element traverses a casing collar, a source ofunidirectional potential of sufficient power output to produce saidminimum current through said firing means, and switch means forselectively connecting said bridge circuit and said unidirectionalsource to the upper end of said cable.

7. A combined casing collar locating and perforating gun firing system,comprising a perforating gun having firing means responsive to apredetermined minimum firing current, a collar locator coil, a singleconductor cable supporting said coil and said gun within a casing forlongitudinal movement therethrough, a bridge circuit at the earthssurface, an isolating transformer, means including the single conductorof said cable and said isolating transformer for connecting said coil inone arm of said bridge circuit, a source of high frequency current forenergizing said bridge circuit, the maximum power output of said sourcebeing less than the power required to produce a current flow over saidcable of sutficient magnitude to excite said firing means, means forindicating unbalance of said bridge circuit resulting from a change inthe impedance of said coil as said coil traverses a casing collar, acontrol circuit at the earths surface, means including the singleconductor of said cable for connecting said control circuit in circuitwith said firing means, and means included insaid control circuit forfiring said firing means.

8. A combined casing collar locating and perforating gun firing system,comprising a perforating gun having firing means responsive to apredetermined minimum firing current, a collar locator coil, a singleconductor cable supporting said coil and said gun within a casing forlongitudinal movement therethrough, a bridge circuit at the earthssurface and having a coupling transformer in one arm thereof, anisolating transformer, means including the single conductor of saidcable and said isolating transformer for elfectively connecting saidcollar locator coil across one winding of said coupling transformer, asource of high frequency alternating current for energizing said bridgecircuit, the maximum power output of said source being insufficient toproduce a current flow through said isolating transformer which is equalin magnitude to said predetermined minimum firing current, means forindicating unbalance of said bridge icrcuit resulting from a change inthe impedance of said coil as said coil traverses a casing collar, apotential source at the earths surface of sufiicient power output toproduce said minimum firing current through said firing means, andswitch means for selectively connecting said coupling transformer andsaid potential source to the upper end of said cable.

9. A combined casing collar locating and perforating gun firing system,comprising a perforating gun having firing means responsive to apredetermined minimum firing current, a collar locator coil, a singleconductor cable supporting said coil and said gun within a casing forlongitudinal movement therethrough, a bridge circuit at the earthssurface and having a coupling transformer in one arm thereof, anisolating transformer, means including the single conductor of saidcable and said isolating transformer for effectively connecting saidcollar locator coil across one winding of said coupling transformer,means connecting said firing means in circuit with said isolatingtransformer, means normally short circuiting said firing means, a sourceof high frequency alternating current for energizing said bridgecircuit, the maximum power output of said source being insufiicient toproduce a current flow through said isolating transformer which is equalin magnitude to said predetermined minimum firing current, means forindicating unbalance of said bridge circuit resulting from a change inthe impedance of said coil as said coil traverses a casing collar, meansat the earths surface for removing the short circuit on said firingmeans, thereby to condition said firing means for detonation, and meansoperative after said short circuit is removed for firing said firingmeans.

10. A combined casing collar locating and perforating gun firing system,comprising a perforating gun having firingmeans responsive to apredetermined minimum firing current, a collar locator coil, a singleconductor cable supporting said coil and said gun within a casing forlongitudinal movement therethrough, a bridge circuit at the earthssurface and having a coupling transformer in one arm thereof, anisolating transformer, a low resistance fuse member, means connectingsaid coupling transformer, the single conductor of said cable, saidisolating transformer and said fuse member in series, means connectingthe firing means of said perforating gun in parallel with said fusemember, a source of high frequency alternating current for energizingsaid bridge circuit, the maximum power output of said source beinginsufiicient to produce a current flow over said cable which is equal inmagnitude to said predetermined minimum firing current, means forindicating unbalance of said bridge circuit resulting from a change inthe impedance of said coil as said coil traverses a casing collar, meansat the earths surface for impressing a current on the single conductorof said cable of sufficient magnitude to burn out said fuse member andclear said firing means, and means at the earths surface for firing saidfiring means, after said fuse member is burned out.

11. A combined casing collar locating and perforating fuse member, meansconnecting said coupling transformer, I

the single conductor of said cable, said isolating transformer and saidfuse member in series, means connecting the firing means of saidperforating gun in parallel with said fuse member, a source of highfrequency alternating current for energizing said bridge circuit, themaximum power output of said source being insuflicient to produce acurrent flow over said cable which is equal in magnitude to saidpredetermined minimum firing current, means for indicating unbalance ofsaid bridge circuit resulting from a change in the impedance of saidcoil as said coil traverses a casing collar, and means at the earthssurface for impressing a current on the single conductor of said cableof sufiicient magnitude to burn out said fuse member and fire saidfiring means.

12. A casing logging system, comprising a locator coil, a cablesupporting said coil within a casing for longitudinal movementtherethrough, a bridge circuit at the earths surface, means includingsaid cable for connecting said locating coil in one arm of said bridgecircuit, a source of alternating current for energizing said bridgecircuit, and rate of change responsive indicating means connected to theoutput of said bridge circuit for indicating only an unbalance thereofhaving a rate of change greater than a predetermined minimum value,whereby said indicating means is substantially nonresponsive to changesin the impedance of said cable as said coil traverses the casing and isresponsive to changes in the impedance of said coil as said coiltraverses a metal-lie anomaly in said casing.

13. A casing logging system, comprising a locator coil, a cablesupporting said coil within a casing for longitudinal movementtherethrough, a bridge circuit at the earths surface, means includingsaid cable for connecting said locating coil in one arm of said bridgecircuit, a source of alternating current for energizing said bridgecircuit, means for rectifying the output of said bridge circuit toprovide a unidirectional control voltage in response to unbalancethereof, and indicating means responsive only to a predetermined minimumrate of change of said control voltage for indicating changes in theimpedance of said coil as said coil traverses a metallic anomaly in saidcasing.

14. A combined casing collar locating and perforating gun firing system,comprising a perforating gun having firing means responsive to apredetermined minimum firing current, an inductive member, a singleconductor cable supporting said inductive element and said gun within acasing for longitudinal movement therethrough, a transformer having onewinding thereof connected in circuit with said firing means and anotherwinding connected to said inductive member, means at the earths surfacefor supplying an alternating current over the single conductor of saidcable to said one winding of said transformer which is smaller than saidpredetermined minimum firing current, said alternating current supplymeans having a maximum power output which is substantially less than thepower required to produce a current flow through said one winding ofsufficient magnitude to excite said firing means, said alternatingcurrent flow through said one winding of said transformer producing asubstantially larger current flow through said inductive member, meansat the earths surface for indicating changes in the impedance of saidinductive member as said member traverses a casing, a substantiallyunidirectional potential source at the earths surface of sufiicientpower to produce said minimum firing current through said firing means,and means for connecting said potential source to said cable in place ofsaid alternating current supply means.

References Cited in the file of this patent UNITED STATES PATENTS1,835,722 Rembert Dec. 8, 1931 12 Crites July 29, 1941 Millington May24, 1949 Pagan June 26, 1951 Swift July 8, 1952

